1. Field of the Invention
The present invention relates to an electrolytic capacitor. More particularly, it relates to an electrolytic capacitor wherein a novel electrolyte is used.
2. Discussion of the Background
An electrolytic capacitor having a capacitor element prepared by rolling foils of a valve metal such as aluminum together with a separator, usually has a structure wherein an electrolyte is impregnated to the capacitor element, and such a capacitor element is accomodated and sealed in a metal casing such as an aluminum casing or in a casing made of a synthetic resin.
In such an electrolytic capacitor, it has been common to employ an electrolyte comprising ethylene glycol as the main solvent and boric acid or an aliphatic dibasic carboxylic acid such as adipic acid or decanedibasic carboxylic acid or an ammonium salt thereof as the solute, as an electrolyte for an intermediate or high voltage capacitor (working voltage of 100 V or higher).
However, when boric acid is used, it is impossible to obtain high conductivity, and when an aliphatic dibasic carboxylic acid or its salt is employed, the deterioration in the properties of a electrolytic capacitor is remarkable under a high temperature condition of at least 100.degree. C. Under the circumstances, it has been studied to employ aromatic carboxylic acids exhibiting excellent properties constantly under a high temperature condition. Among them, benzoic acid imparting a relatively high sparking voltage is suitable as a solute for an electrolyte for an intermediate voltage electrolytic capacitor, and an electrolyte having ammonium benzoate dissolved in ethylene glycol is known (Japanese Examined Patent Publication No. 8501/1977).
However, to secure a long useful life of an electrolytic capacitor under a high temperature condition, it is desirable not only to adjust the pH of the electrolyte to a level of from 5 to 7, but also to minimize the water content. If the pH is high or the water content is large, the electrode foils are likely to be corroded, whereby the properties of the capacitor tend to deteriorate, or a gas is likely to be generated to actuate a safety valve. On the other hand, if the water content is limited to a level of not larger than 5% or the pH is limited within a range of from 5 to 7, it will be impossible to obtain high conductivity with benzoic acid alone, whereby it is difficult to obtain a high performance electrolytic capacitor having a small loss (tan .delta.). Further, it is also known to use ammonium benzoate and ammonium succinate in combination (Japanese Examined Patent Publication No. 9340/1968). Succinic acid is durable up to a temperature of 85.degree. C. However, when the temperature rises to 105.degree. C., succinic acid undergoes thermal decomposition, and even if it is used in combination with benzoic acid, it is not durable for use for a long period of time. An electrolyte is also known wherein ammonium benzoate, boric acid and mannitol are dissolved in ethylene glycol as the solvent (Japanese Unexamined Patent Publication No. 60829/1982). The electrolyte of this system is said to show stability properties under a high temperature condition at a level of from 130.degree. to 150.degree. C., but it is difficult to obtain sufficiently high conductivity, and it is impossible to obtain an electrolytic capacitor having an adequately small loss (tan .delta.).